Acoustic wave devices have been used extensively in the art as frequency reference resonators, delay lines, and sensors. The oldest acoustic wave device structure is the parallel plate resonator, which consists of a plate of piezoelectric material having substantially flat and parallel polished surfaces, one or both of which support one or more conducting electrodes. When a voltage signal is applied between the electrodes, stress fields induce elastic deformations of the crystal (strain fields). The deformations of the crystal alter the distribution of charge within the crystal and a net flow of charge (a current) exists.
A more advanced acoustic wave device utilizes surface acoustic waves, surface transverse waves, or acoustic plate modes. Those devices are generally known as SAW devices, or as acoustic plate mode devices. Briefly, these devices comprise a substrate of piezoelectric material such as quartz or lithium niobate, or thin films of piezoelectric material, such as zinc oxide, or cadmium sulfide, on a non-piezoelectric substrate. The substrate has at least one active piezoelectric surface area, which is highly polished. Formed on the surface are input and output transducers for the purpose of converting input electrical energy to acoustic energy within the substrate and reconverting the acoustic energy to an electric output signal. The input and output transducers frequently comprise interdigitated transducers each comprising a plurality of interdigitated electrode fingers which are electrically coupled to an input signal, and to an output measurement device respectively. Such transducers are known as IDT (Inter Digitated Transducer) and are typically formed by depositing a thin film of electrically conductive material such as aluminum or gold in the desired shape on the active area. Electrical potential is coupled to the input transducer and induces mechanical stresses in the piezoelectric substrate. The resultant strains propagate along the surface of the substrate to the output transducer, whereby they are converted to output electrical signals. The waves may propagate along the surface of the crystal (surface modes), or through the bulk of the crystal structure (waveguide modes).
When designing an acoustic wave device, one has to consider the size, number, mass, shape, and connection method of the electrodes, as those parameters significantly effect the behavior of the device. The effects of the electrode design are known in the art. However, for simplicity, these specifications will relate to electromechanically insignificant electrode structure to mean that an acoustic wave traveling under a short-circuited transducer containing electromechanically insignificant electrodes, would experience no significant reflective coupling into a reverse-traveling wave due to the periodic perturbations from the nominal surface conditions outside the transducer region. The opposite of electromechanically insignificant electrode structure described above, is naturally the electromechanically significant electrode structure, which means that such reflective coupling would be created and enhanced.
Typically, insignificant electrodes are designed to have a minimum thickness (mass) required by the electrode dimensions to provide adequate conductivity and the ability to affix bond wires. However, as the operating frequency increases, creating mechanically insignificant electrodes becomes harder as the dimensions decrease inversely to the frequency. Furthermore, in piezoelectric materials such as lithium niobate having high piezoelectric coupling, mechanically insignificant electrodes are electrically (and thus electromechanically) significant. However a method of connecting more than one consecutive electrode to the same polarity is known to locally cancel the resulting electromechanical reflections and thus make the electrodes effectively be electromechanically insignificant, even at higher frequencies or in high coupling materials.
It is clear that as the acoustic wave propagation in the crystal is mechanical in nature, changing the environment in which the crystal operates changes the behavior of the crystal, and it follows that the electrical characteristics exhibited by the piezoelectric device change as well. Thus for example, temperature, and/or the medium in which the crystal is suspended, varies the crystal response. Therefore, exposing a driven piezoelectric crystal to contact with a fluid or a polymer material will dampen the wave propagation in the crystal, and change the characteristic response of the device. This change may be used to measure certain characteristics of the fluid, or may be a side effect of polymer films in gas sensing or of the fluid in efforts to measure other properties of said fluid. In this manner a sensitive sensor is created. Sensors may also be made responsive to specific substances. This is done by for example, placing a polymer film on the sensor. If the polymer is sensitive to a specific gas or biological agent for example, the polymer characteristics will change, and with it its effect on the mechanical perturbations in the crystal. Examples of such sensors is taught in U.S. Pat. No. 5,283,037 to Baer et al., in U.S. Pat. No. 5,235,235 to Martin et al., and U.S. Pat. No. 6,033,852b to Andle et al.
The ideal sensor will have a narrow bandwidth, high Q factor, and a well defined behavior change in response to the measured parameter. It will exhibit wide dynamic range, be energy efficient, (both for efficiency's sake, and to limit the need for high amplification with its inherent problems), and operate predictably over a wide range of variation in the ambient environment. Clearly, the sensor must have unique, measurable changes throughout the measurement range, which typically should be as wide as possible.
Feeding an alternating voltage signal into the input transducer of an acoustic wave device will cause periodic deformations in the crystal, and the generated acoustic waves, when incident on the output transducer, will cause a net current effect into the transducer's load impedance. Using a low impedance current measuring device, it is possible to measure the delayed and attenuated replica of an input voltage signal at the output transducer. In an ideal device, a signal introduced into the acoustic wave device will traverse the device only once. However, as the transducers and their respective circuits are not ideal, the current produced in the output transducer, unless delivered to a short-circuit load, will cause regeneration of the input signal, which will cause a regeneration of the signal at the input transducer, and so forth ad infinitum. This reflective phenomenon hinders the design of devices such as acoustic wave delay lines, while other devices such as resonators, take advantage of it.
A typical acoustic wave delay line 100 as depicted schematically in FIG. 1, comprises a piezoelectric substrate 105, an input 110 and output 130 transducers deposited on the substrate, and separated by a relatively long passive propagation path 120. As the signal in the output transducer is delayed by the time it takes the periodic deformations to propagate in the crystal, a delay line is formed. The ideal delay line will exhibit a broad bandwidth and minimized reflection, preferably only a single transit between the input and output transducers, in order to achieve the finite impulse response desired from such device. Therefore in order to minimize the ripple and reflections, design criteria calls for making the delay line utilizing the smallest number of electrodes necessary for the desired electrical coupling efficiency, and making the electrodes as small, light, and electrically insignificant as possible, in order to minimize their reflective effects.
Common wisdom in the art also dictates that a very low ripple is an important design goal for delay lines. For example, in the popular book in the field “Surface Wave Filters, Design Construction, and Use”, to Mathews, (page 153, Wiley Interscience Publication, John Wiley & Sons, New York, USA) less than ±0.5 dB is desired. Other authorities in the field also indicate this stated design goal, which dictates the use of electromechanically insignificant electrodes. Large number of electrodes or the use of electromechanically significant electrodes, causes triple transit echoes, and also another problem considered undesirable by those skilled in the art, (see Matthews page 156, line 3) of “ - - - reflections within a given transducer - - - ”. Thus the present state of the art generally teaches that the reflections are significant with as little as 10 electrodes on lithium niobate or 100 electrodes on quartz. Accepted design practices call for reducing the electrodes even below those numbers, to a bare minimum required for appropriate coupling, and reducing the electromechanical significance of the electrode.
In order to further reduce ripple and regeneration effects while using a delay line in a phase coherent circuit, common practice requires a mismatch between the input and output impedances and their respective electrical circuits that will cause an insertion loss of about 20 db between the input and on the output, even in the absence of damping effects. Other methods of reducing the regeneration effects have been taught by using unidirectional transducers often implemented by inserting a reflector, typically made of grating of either metal electrodes or of slots cut out in the piezoelectric substrate, which are designed to be in 180 degrees phase shift of the regenerated signal and thus to cancel it. The phase coherent delay line using bi-directional transducers suffers from low efficiency, which limits the availability of dynamic range needed for sensor applications. The unidirectional transducers are dependent on a critical balance of mechanical reflections and electrical regeneration, which is difficult to maintain over the variations in electrical parameters and insertion loss dynamic range needed for sensor applications. This is, in part, a result of the variable mechanical damping involved, which alters both the required point of balance between reflection and regeneration and the amount of reflection available to balance the regeneration.
From the above discussion it is seen that if the ripple in an acoustic wave device operating as a delay line, can be controlled by the device structure rather than by the external circuitry connected to it, the device will be better suited for sensor applications.
Resonators represent another common acoustic wave device type. The resonator is commonly used in oscillator circuits as the timing element in the feedback loop of an oscillator, and similar circuits. Thus the electrical characteristics desired in a resonator call for a very sharp frequency bandwidth, and a very high Q to allow for efficient coupling. As shown in FIG. 2, the typical acoustic wave resonator comprises a piezoelectric crystal, with relatively short, low electrode count, input 200 and output 210 transducers deposited on the crystal. A relatively short resonant cavity 220 is interposed between the transducers. In order to absorb most acoustic waves but enhance the reflection of acoustic waves that are at the resonator frequency, extensive regions of electromechanically significant reflective gratings 230 and 240 are added to the sides of the resonant cavity to act as tuned signal mirrors. The acoustic wave that is trapped between these tuned grating reflectors is multiply reflected with a long propagation path between reflections, and the echoes produce a high Q factor resonance. While most reflections do not add coherently, a standing wave will be created at one or more frequencies of coherent reflection that depends on the grating's relationship to the wavelength. The input transducer drives the standing wave and the output transducer detects that standing wave. Thus the resonator exhibits high energy efficiency, (a high Q factor due to the infinite impulse response), and a very narrow, resonant spike type bandwidth. It should be noted that some resonator design call for a single transducer, and some call for mixing the transducers within the reflective grating (U.S. Pat. No. 4,144,507 to Shreve), but the general behavior and operational principle of the resonator remains the same.
The high Q factor exhibited in a resonator is very desirable for sensor applications. However, when dampening is applied to the conventional resonator, such as by exposing it to a liquid, or depositing a film thereupon, the signal decays rapidly, before it can be reflected back into the transducers a sufficient number of times. Dampening therefore turns the high Q factor resonator in free air, into a very low Q factor device when coated or suspended in other fluids. Because a resonator degrades very rapidly upon dampening from external effects, it is of limited use in sensor applications having substantial damping of acoustic waves.
Additionally, loading a resonator by the like of a polymer film, significantly raises the resonator's typically low insertion loss, often increasing from approximately 7 dB to approximately 20 dB with even light damping. Clearly this causes a reduction in the dynamic range available for a sensor application. This effect has even been observed for relatively high acoustic quality passivation films of silica or silicon nitride.
There is therefore a clear and heretofore unanswered need for an acoustic wave sensor structure which exhibits narrow bandwidth and that offers high energy efficiency without the severe degradation exhibited by the existing structures. The present invention aims to provide such a device.